BUNSEW^ MEMORIAL LECTURE. (525 



The original views of Bunsen and Kirchhotf coneer.mur the nature 

 ot the spectra of the alkali and alkaline-oarth metals as examined in 

 the flame of the Bunsen burner has since their time undercr<,n,M-nn- 

 siderable modification. We now know that while the sp,Mtrum of 

 potassmm, sodmm, caesium, rulndium, and lithium, producvd when 

 any compound of these elements is hrouoht into the flame, is that (,f 

 the metal, it is quite otherwise with the similar spectra of the aii<aline 

 earths, for if a bead of calcium, strontium, or })arium salt ]>v hrou.rht 

 into the flame bright lines and bands are seen, characteristic indeed 

 of the individual substance, but difl'ering altogether from the spectra 

 obtained from the above compounds at the high temperature of the 

 electric spark. In the first case Ave are dealing with the spi-ctra of a 

 compound, whereas in the latter instance we obtain the line spectrum 

 of the metal itself. Nor must it be forgotten that Bunsen was the 

 first to point out that which has only in recent years been fullv 

 recognized, namely, that change of physical condition undei- which a 

 spectrum is observed may give rise to fundamental changes in the 

 character of the spectrum itself. It was in his research on the 

 absorption spectrum of didymium (Pogg. Ann., 1866 (128), loo), 

 carried out with minute care, that this point was made clear. In / 



this he proved that, examined under a high dispersive and magni- / 

 fying power, a crystal of didymium sulphate gives an abs()rption 

 spectrum in which the dark bands var}- in position and in breadth 

 according to the position of the crystal in regard to its axes through 

 which the light passes; that is, whether the polarized ray is ordinary 

 or extraordinary. These changes, somewhat similar to thos*' since 

 shown to be effected by change of pressure undei- magnetic influence, 

 or from change of temperature, have 3'et to receive a satisfactory 

 explanation. To enlarge upon these matters is, however, beyond the 

 province of the present address, sutfice it to say that Bunsen's original 

 investigation has opened out an unbounded field for research, the ml 

 tivation of which has alread}^ yielded great results and will in future 

 yield still greater ones. 



Next let us turn to his celebrated researches on cheniic:il geology, 

 especially those concerning the volcanic phenonn'iia of Iceland. 



The only relaxation from his scientific labors which Hunsen through- 

 out life allowed himself was traveling, and this he tlioiouL-hly enjoyed. 

 During many autumn vacations 1 had the ])leasure of acconiiKinying 

 him in rambles throughout Switzerland and the Tyi-ol. He walked 

 well and had a keen appreciation of natural beauty. esi)ecially of 

 mountain and woodland scenery, while he took great interest in the 

 geology and physical characteristics of the districts through which 

 he passed, and this it was that led him to turn his mind to cheniico: 

 geological studies. So early as 1844, in company with I'illa :in<l Mat- 

 teucci, he visited and carefully examined the L'arboniferous .lei...sit« 



SM 99 40 



